Re: [PATCH v3 16/17] sparse-checkout: write using lockfile

>
> From: Derrick Stolee <dstolee@microsoft.com>
>
> If two 'git sparse-checkout set' subcommands are launched at the
> same time, the behavior can be unexpected as they compete to write
> the sparse-checkout file and update the working directory.
>
> Take a lockfile around the writes to the sparse-checkout file. In
> addition, acquire this lock around the working directory update
> to avoid two commands updating the working directory in different
> ways.

>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  builtin/sparse-checkout.c          | 15 ++++++++++++---
>  t/t1091-sparse-checkout-builtin.sh |  7 +++++++
>  2 files changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/builtin/sparse-checkout.c b/builtin/sparse-checkout.c
> index 542d57fac6..9b313093cd 100644
> --- a/builtin/sparse-checkout.c
> +++ b/builtin/sparse-checkout.c
> @@ -308,6 +308,8 @@ static int write_patterns_and_update(struct pattern_list *pl)
>  {
>         char *sparse_filename;
>         FILE *fp;
> +       int fd;
> +       struct lock_file lk = LOCK_INIT;
>         int result;
>
>         if (!core_apply_sparse_checkout) {
> @@ -317,21 +319,28 @@ static int write_patterns_and_update(struct pattern_list *pl)
>
>         result = update_working_directory(pl);
>
> +       sparse_filename = get_sparse_checkout_filename();
> +       fd = hold_lock_file_for_update(&lk, sparse_filename,
> +                                     LOCK_DIE_ON_ERROR);
> +
> +       result = update_working_directory(pl);
>         if (result) {
> +               rollback_lock_file(&lk);
> +               free(sparse_filename);
>                 clear_pattern_list(pl);
>                 update_working_directory(NULL);
>                 return result;
>         }
>
> -       sparse_filename = get_sparse_checkout_filename();
> -       fp = fopen(sparse_filename, "w");
> +       fp = fdopen(fd, "w");
>
>         if (core_sparse_checkout_cone)
>                 write_cone_to_file(fp, pl);
>         else
>                 write_patterns_to_file(fp, pl);
>
> -       fclose(fp);
> +       fflush(fp);
> +       commit_lock_file(&lk);
>
>         free(sparse_filename);
>         clear_pattern_list(pl);
> diff --git a/t/t1091-sparse-checkout-builtin.sh b/t/t1091-sparse-checkout-builtin.sh
> index 82eb5fb2f8..f22a4afbea 100755
> --- a/t/t1091-sparse-checkout-builtin.sh
> +++ b/t/t1091-sparse-checkout-builtin.sh
> @@ -262,4 +262,11 @@ test_expect_success 'revert to old sparse-checkout on bad update' '
>         test_cmp dir expect
>  '
>
> +test_expect_success 'fail when lock is taken' '
> +       test_when_finished rm -rf repo/.git/info/sparse-checkout.lock &&
> +       touch repo/.git/info/sparse-checkout.lock &&
> +       test_must_fail git -C repo sparse-checkout set deep 2>err &&
> +       test_i18ngrep "File exists" err
> +'
> +
>  test_done
> --
> gitgitgadget
>